The present invention relates to the field of optical frequency harmonic generation, and in particular to optical fourth-harmonic generation.
Short wavelength (e.g., ultraviolet) lasers find use in drilling, microwelding, micromachining, component trimming, glass and semiconductor marking, rapid prototyping, electronic packaging, corneal sculpturing, cardiac surgery to give blood flow to heart muscles, fluorescence spectroscopy, including time resolved techniques. The relative lack of availability of short-wavelength lasers has fueled much of the research effort to develop radiation sources based on frequency conversion. Radiation sources based on second- third- and fourth-harmonic generation have been proposed for producing coherent ultraviolet (UV) radiation.
In a conventional system based on third-harmonic generation, radiation of a fundamental (xcfx89) frequency is used in a -non-linear crystal to produce radiation of a second-harmonic (2xcfx89) frequency. The second harmonic radiation is mixed with the fundamental radiation, typically in a different non-linear crystal, to produce radiation of a third-harmonic (3xcfx89) frequency. The third-harmonic radiation (a useful output of the system) may then be physically separated from the fundamental, second-harmonic and third-harmonic. radiation, e.g., using an intracavity or extracavity, element such as any prism, a grating, a Brewster prism or a dichroic-coated mirror. Because of the relative inefficiencies of the three frequency-conversion steps, the intensity of the third-harmonic-radiation may be much lower than that of the fundamental radiation. Consequently, third-harmonic systems are extremely sensitive to optical losses, and in particular to losses to the fundamental radiation.
The second- and third-harmonic crystals are usually anti-reflection coated to prevent back reflections from the input and output surfaces of the crystals. Optical coatings are generally very, sensitive to optical-damage, however, and in particular to damage, caused by short-wavelength radiation. An arrangement allowing the use of optically-uncoated crystal surfaces while simultaneously providing for wavelength separation and avoiding the back-reflection problem would be of great value in a radiation source based on third-harmonic generation. Such an arrangement would be particularly desirable for a system using intracavity frequency conversion element(s).
Prior art third-harmonic system for generating ultraviolet (355 nm) radiation from infrared (1064 nm) radiation have used lithium triborate (LBO) crystals for frequency doubling and tripling in a resonant cavity containing a Nd-YAG solid state laser. Brewster-cut intracavity prisms have been used to separate the ultraviolet beam from the fundamental beam.
Although intracavity prisms may reduce losses relative to beam separation schemes using dichroic mirrors, the intracavity prism tends to increase the complexity of the system. Furthermore, intracavity prisms do not alleviate the need for anti-reflection coating the output surface of the frequency-tripling crystal in order to minimize losses to the fundamental and third-harmonic radiation beams. Leaving the output surface uncoated in such prior art systems would result in high losses to the fundamental radiation. At the same time, the use of an AR coating for the output surface severely limits the useful lifetime of the system, due to catastrophic or long-term UV-induced damage to the AR coating.
Commonly assigned U.S. Pat. No. 5,850,407 describes a third-harmonic generator system that uses a second-harmonic generating crystal coupled to a third-harmonic generating crystal. The third-harmonic generating crystal has an uncoated dispersive output facet. The output facet is preferably oriented at Brewster""s angle with respect to the fundamental and third harmonic radiation, such that the output facet does not substantially impede the passage of fundamental or third harmonic radiation. In addition, the output facet impedes the passage of any s-polarized component of radiation, thus acting as a polarization-selective element. The dispersive output facet spatially separates fundamental radiation of frequency xcfx89 from third-harmonic radiation of frequency 3xcfx89. In this system, both the fundamental and third-harmonic radiation are p-polarized with respect to the output facet.
Non-linear materials have also been used to produce radiation of a fourth-harmonic of the fundamental. For example, the third-harmonic radiation may be mixed with the fundamental radiation, typically in another different non-linear crystal, to produce radiation of a fourth-harmonic (4xcfx89) frequency. Unfortunately, attempts to produce a suitably configured fourth-harmonic generator system have been unsatisfactory, particularly for multi-kilohertz systems.
Early attempts used a first non-linear material, e.g., to generate 532 nm second-harmonic radiation from 1.064-micron fundamental radiation. A second non-linear material generated 266 nm fourth-harmonic from the 532 nm second-harmonic radiation by a frequency doubling interaction. Unfortunately, the 532 nm to 266 nm frequency doubling interaction does not phase match in LBO, which is a most robust and desirable non-linear material. KDP and its isomorphs have been used successfully in low repetition rate joule-class systems but not in multi-kilohertz systems or lower energy systems. Other prior art attempts have utilized three non-linear crystals. The first crystal doubles 1.064-micron fundamental radiation to generate 532 nm second-harmonic radiation. The second crystal sums a portion of the fundamental radiation with the second-harmonic radiation to produce 355 nm third-harmonic radiation. The third crystal sums a portion of the fundamental radiation with the third-harmonic radiation to produce 266 nm fourth-harmonic radiation. If the fundamental and third-harmonic are polarized in the same state then the fourth-harmonic is orthogonally polarized with respect to fundamental and third-harmonic. Hence, if the fundamental radiation is p-polarized with respect to a Brewster prism surface then the fourth-harmonic is unfortunately s-polarized. As a result, it is necessary to either rotate the polarization of the fourth-harmonic radiation to p-polarization in order to meet the Brewster""s angle condition at the output facet of the fourth harmonic generator crystal or anti-reflection (AR) coat the output facet. It is extremely difficult to rotate the polarization of the fourth-harmonic radiation without either compromising the fourth-harmonic phase matching or making the system more complex, e.g., by bonding a polarization rotating region onto the fourth-harmonic generator crystal. Techniques involving harmonically-selected waveplates, etc. require AR coatings. AR coatings are highly susceptible to damage at UV frequencies, which reduces the useful lifetime of system components. Thus it has been difficult to meet the Brewster angle condition for p-polarized fundamental and fourth-harmonic radiation while phase matching the radiation to the fourth-harmonic-generating crystal without using AR coated surfaces and/or Brewster prisms.
Thus, there is a need in the art, for a fourth-harmonic generating system and method that overcomes the above disadvantages.